SUMMARY OF WORK: We have demonstrated that, among a large array of Drosophila melanogaster strains (including those bearing various mutant allels at the white locus), only those bearing the wa allele are labeled at the cytological location of the white locus after in situ hybridization using as probe a cloned copy of an interspersed, reiterated DNA sequence. Most importantly a wBwxwa double mutant (wBwx is a white mutant allele mapping to the left of wa) and a wawch double mutant (wch is a white mutant allele mapping to the right of wa allele) are labeled at white in this way while neither the wBwx nor the xch allele alone is labeled. These results suggest that the wa mutation results from the insertion of a copy of the cloned DNA sequnce element into the white region DNA seqences.